Pipes used, for example, in oilfield pipelines, have been joined end-to-end to transport oil or other fluids from place to place. Typically, the ends of the pipes are joined by threading together the male end of one of the pipes, known as a pin, with the female end of the other pipe (known as the box). There are a number of different types of connections. Some connections are known as integral flush connections, wherein the connection is machined into the pipe body, without adding any additional material, or up-setting, when joining the two pieces of pipe. The result is a connection having an outer diameter that is flush with the outer diameter of the pipes, and an inner diameter that is flush with the inner diameter of the pipes. Most integral flush connections have a tension efficiency of about 68% to about 72%.
The term “tension efficiency,” which is sometimes also referred to as “tensile efficiency,” generally refers to the tensile strength of the threaded connection divided by the tensile strength of the pipe body itself. Furthermore, the term “tension efficiency” may also be used to refer to the critical cross-sectional area divided by the area of the pipe body. One reason that tension efficiency is limited to about the range described above, is because when an integral flush connection is machined, material is cut away from the pipe end to form the flush connection, which leads to a thinner pipe at the connection compared to the rest of the pipe body.
In threaded connections, there is typically a weak point, which is the point where the connection is most likely to fail. This weak point is typically the last engaged thread of the connection, also referred to as the “critical cross section.” On a flush connection, the critical cross section is most always on the box. One way to increase the strength of a connection, therefore, is to minimize the stresses in the last engaged thread.